1. Field of the Invention
The present invention relates to homopolar motor-generators, and more particularly to an improved homopolar motor-generator that does not require sliding electrical contacts upon its high linear speed surfaces.
2. Description of the Prior Art
A homopolar motor-generator produces or utilizes direct electric current without resorting to a commutator; it is a true direct current device. The machine is a current rather than a voltage device, producing large currents at low voltages. Historically it has been employed as a ship, locomotive and urban transit drive because it yields full torque at zero speed. It has also been used as a direct current source for both large and small scale welding installations and an energy source for metallurgical heat engineering.
In the past two decades the homopolar machine has gradually become a useful tool in the "high technology" theater. For example, they have served as an energy store, often called "mechanical capacitors," for electromagnetic launchers; as a power supply for pinch and shock wave production in thermonuclear installations; and, as magnet drivers for fusion research Commercially it is employed as a large section butt welder.
The progress and development of homopolar motor-generator technology has been impeded by the requirement for high speed sliding contacts. Such contacts are provided at the periphery of a disc rotor turning on an axle in a magnetic field. A plurality of sliding contacts extract current from the periphery of the rotor and return the current to the axle. Such designs have been limited to speeds of about 200 meters per second. This limitation is discussed in Weldon, W. F., "Pulsed Power Packs a Punch," IEEE Spectrum, March, 1985. Other authorities quote speeds under 100 meters per second as the practical limit. The difficulties of gathering current from high-speed conductors are quantified in E. I. Shobert, Carbon Brushes--The Physics and Chemistry of Sliding Contacts, (Chemical Publishing Co., New York, 1965).
The limitations and troubles that arise from the sliding contacts are not new. They were fully recognized by the machine's inventor, Michael Faraday. He saw the device's efficiency destroyed by the resistance across the sliding contacts. Faraday attempted to solve the problem with liquid metal contacts using mercury. Today's researchers assault the problem with a variety of techniques ranging from water cooled silver-graphite brushes operating in a special gas environment to complex liquid sodium-potassium metal bath brushes.
One design to eliminate the sliding contacts of the homopolar machine is presented in H. D. Varadarajan, "Planetary Homopolar Generator," IBM Technical Disclosure Bulletin, Vol. 17, No. 6, p. 1786-1787, Nov., 1974. The proposed homopolar machine uses a conducting belt or rolling contacts to gather current from a rolling magnetic flux cutting rotor, executing a planetary motion within an annular magnetic field. A first disadvantage of the machine is managing the live loads and large stresses resulting from the centrifugal force of the massive, and unbalanced, planetary rotor. Furthermore, the rate of magnetic flux cutting of the machine's rotor is low, compared to ordinary prior art techniques, because of the slow rotor rotation resulting from the planetary motion. Low rates of magnetic flux cutting imply low output voltages. If high rates of magnetic flux cutting are contemplated by increasing the planetary velocity of the rotor, then the first disadvantage is exacerbated.
As indicated, conventional homopolar motor-generators comprise a disc rotor and a plurality of sliding contacts mounted at the periphery of the disc and riding thereon. Such sliding contacts, usually carbon brushes, are disposed to gather current from the rim of the homopolar's rotor. In practice, the entire rim surface, on the order of square feet, is covered with spring loaded brushes. The resulting drag on the rotor is large, and in turn leads to heating, wear, noise, and maintenance. In high power operations, either continuous or pulsed mode, the brushes produce arc erosion and metal vapor, damaging the rotor surface, and are a source of expensive repair.
Accordingly, there is a need for improved homopolar motor-generators which are not limited by sliding contacts. The present invention satisfies that need.